Dozens of interviews, court filings and other documents show Mr. Kilimnik to be an operator who moved easily between Russian, Ukrainian and American patrons, playing one off the other while leaving a jumble of conflicting suspicions in his wake. The effort to disentangle the mysteries surrounding him seems likely to leave questions even after the conclusion of the special counsel’s work.

To American diplomats in Washington and Kiev, he has been a well-known character for nearly a decade, developing a reputation as a broker of valuable information like the alliances of Ukraine’s oligarchs and the country’s handling of foreign investment and sensitive criminal cases.

.. Prosecutors have also scrutinized the effort by Mr. Manafort and Mr. Kilimnik to drum up political consulting business with Kremlin-aligned political figures in Ukraine and Russia who were pushing plans to end the simmering conflict between the countries.

Those so-called peace plans could have resulted in the easing of sanctions imposed on Russia by the United States — a policy shift to which Mr. Trump had signaled an openness during the campaign and one that would have been a major foreign policy victory for the Kremlin.

.. To some of those he encountered, he was an impish, 5-foot-tall cynic whose American associates nicknamed him “Carry-on” or “KK” and who was rejected for a job with an oil company in Moscow in late 2003 or early 2004 because he was seen as too meek.

At the same time, he did little to defuse long-running suspicions that he was a Russian agent. And his involvement in discussions related to back-channel peace negotiations between Ukraine and Russia attracted attention from President Barack Obama’s National Security Council, which saw him as a functionary for oligarchs working to sell out Ukraine to Moscow’s benefit, a former United States official said.

Mr. Kilimnik, 49, who has Russian citizenship, now lives in Moscow. He is unlikely to ever face obstruction of justice charges that the special counsel brought against him and Mr. Manafort. And the once chatty operative — who was known for kibitzing with reporters, diplomats and political consultants over WhatsApp and in the bar of the Hyatt hotel in Kiev — has gone dark.

“There was talk of him being related to Russian intelligence agencies going as far back as the 1990s,” said Michael R. Caputo, who traveled in the same circles as Mr. Kilimnik when both worked in the Moscow offices of American pro-democracy groups in the years after the fall of the Soviet Union.

In Kiev, Mr. Kilimnik became a valued source for the political staff of the United States Embassy, because he did not try to sugarcoat the financial motivations of the oligarchs who funded the political parties for which he worked, said David A. Merkel, who handled Ukraine issues as a deputy assistant secretary of state for President George W. Bush.

“The idea that he is some master spy seems hard to fathom,” said Mr. Merkel, who was Mr. Kilimnik’s boss at one of the pro-democracy groups. “I find it much more likely that these guys were pursuing business interests without regard to core patriotic beliefs.”

.. Already fluent in Russian and Ukrainian, he studied Swedish and English at a Soviet military language academy considered a training ground for the country’s military intelligence service, known as the G.R.U. He joined the Russian Army as a translator, and said he later worked for a time in Sweden for a Russian arms exporting company.

In the chaotic years after the Soviet Union collapsed, Mr. Kilimnik was left without steady income. He made do with money from freelance translating gigs, before landing a job in 1995 in the Moscow office of the International Republican Institute, or I.R.I., which receives tens of millions of dollars annually from the United States government to promote democracy.

At the institute, Mr. Kilimnik came to know a handful of brash young Americans who would eventually become key players in the Manafort milieu and the Russia investigations, including Mr. Caputo, Philip M. Griffin and Sam Patten.

It was Mr. Griffin who initially served as Mr. Manafort’s right-hand man in Kiev. He first brought Mr. Kilimnik onto the team, only to watch Mr. Kilimnik encroach on his position and eventually formally replace him when Mr. Griffin left the team in 2011. Years later, Mr. Kilimnik went into business with Mr. Patten, advising a Russia-aligned Ukrainian party that had once been Mr. Manafort’s client. The arrangement brought Mr. Patten to the attention of the special counsel’s team, and he pleaded guilty last year to charges related to his lobbying with Mr. Kilimnik.

Mr. Merkel, who led the institute’s Moscow office when Mr. Kilimnik came on board, said Mr. Kilimnik “took the job for the money. Not because he believed in the mission of I.R.I., or in advancing the principle of free-market democracy.”

Mr. Merkel said he later arranged an interview for Mr. Kilimnik at TNK-BP, a joint Russian-British oil venture based in Moscow. “They didn’t think he was tough enough to handle the rough and tumble of Moscow at the time,” Mr. Merkel said. “He hadn’t become what he would later become.”

While his I.R.I. colleagues said Mr. Kilimnik seemed to be no fan of the fallen Communist government or Russia’s post-Soviet leadership, he appeared to embrace the perception that he had ties to Russian intelligence.

“He did nothing to disabuse that notion because it added to his back story and made him more attractive to visiting consultants and women,” Mr. Caputo said.

Two former I.R.I. colleagues have said the group fired Mr. Kilimnik in April 2005 after suspicions arose that he had leaked information about an institute conference in Bratislava, Slovakia, to Russia’s Federal Security Service, the successor agency to the K.G.B.

An institute spokeswoman declined to address the allegation, instead saying that Mr. Kilimnik was fired because the organization “came into possession of information that led us to believe that he was joining Paul Manafort’s team on the Ukraine program.”

Mr. Kilimnik quickly made himself indispensable to Mr. Manafort, who spoke neither Ukrainian nor Russian. At the time, Mr. Manafort worked as a political and business consultant to the Ukrainian steel and coal oligarch Rinat L. Akhmetov, as well as the Russian aluminum oligarch Oleg V. Deripaska.

Mr. Kilimnik traveled extensively with Mr. Manafort, accompanying him to all his meetings and eventually becoming known as “Manafort’s Manafort” in Kiev.

“Paul took Konstantin under his wing, not just as a protégé but as his own surrogate son,” Mr. Caputo said.

Mr. Gates, who joined Mr. Manafort’s Ukraine team the year after Mr. Kilimnik and has since pleaded guilty to charges brought by Mr. Mueller stemming from his work there, later told an associate he knew that Mr. Kilimnik was a former officer with the G.R.U., according to a court filing by Mr. Mueller’s team.

In a February 2017 interview with The New York Times in Kiev, Mr. Kilimnik denied ever serving in a Russian intelligence agency. “What would I do if I were a real Russian spy?” he said. “I would not be here. I would be in Russia.”

But for the politicians and oligarchs who were Mr. Manafort’s clients, Mr. Kilimnik’s suspected intelligence connections suggested a seal of approval from Moscow. That was an important selling point, especially when combined with Mr. Manafort’s connections. The perception in political circles in Kiev was that hiring Mr. Manafort’s team would open doors in Washington and Moscow.

Mr. Akhmetov persuaded Mr. Manafort to try to resuscitate the political career of the former Ukrainian prime minister Viktor F. Yanukovych, a Russia-aligned figure who lost the 2004 presidential election amid allegations of vote-rigging.

In Mr. Yanukovych’s Party of Regions, Mr. Kilimnik was “known as the representative of Russia,” said Taras V. Chernovyl, a former party member.

With help from Mr. Manafort and Mr. Kilimnik, Mr. Yanukovych won the presidency in 2010.

Mr. Manafort, who was not known for spreading the wealth among his subordinates, paid Mr. Kilimnik $530,000 for “professional services and administrative overhead for Kiev operations” from June 2013 to January 2014, according to Justice Departmentlobbying filings, which show that Mr. Manafort’s company was paid $17 million over a two-year stretch around that time.

.. By then, Mr. Kilimnik was married with two children. His family lived in a modest house not far from Sheremetyevo International Airport in Moscow, where he traveled most weekends. But he lived a parallel life in Kiev, where he embraced trappings of his boss’s jet-setting lifestyle, trading the khakis and sweaters he wore at I.R.I. for tailored suits, a chauffeur-driven German car and evenings at an estate with a pool.

Things started going south for Mr. Yanukovych and, by extension, Mr. Manafort and Mr. Kilimnik, in late 2013, as mass protests erupted over the government’s corruption and pivot toward Russia. Mr. Yanukovych stepped down and fled in February 2014, eventually arriving in Moscow.

Russia began a military incursion into eastern Ukraine in 2014. And Mr. Kilimnik leveraged his access to American diplomats for a new endeavor.

He turned up in Donetsk as the city was slipping into war, presenting himself as having “the State Department’s ear, the American ear in general,” said Aleksei Kovzhun, a member of a group of pro-Ukrainian political advisers working there.

.. Mr. Kilimnik joined their ranks, and in emails reviewed by The Times, he encouraged them to appease the Russian-speaking population, who, he wrote, merely wanted “stable work, good salaries and a turn to the better for their children.”

The American Embassy was overly focused on the Russian military threat, he said. “The whole American Embassy is counting barricades,” he wrote, adding about the United States diplomats he was in contact with, “the idiots.”

In hindsight, Mr. Kovzhun said, “I can see that he could easily be a Russian agent who just came to monitor the situation, and be in the center of pro-Ukrainian power in the region.”

With their primary benefactor in exile, Mr. Manafort and Mr. Kilimnik sought business with Russia-aligned factions that arose from the ashes of Mr. Yanukovych’s party.

A common goal for these factions was to pursue initiatives to settle the Ukraine conflict on terms seen by many in the country as favorable to Russia, and as an opening for an effort to persuade the United States and its allies to lift sanctions on Moscow.

Among the Russia-aligned Ukrainians they advised was Oleksandr V. Klymenko, who was a minister in Mr. Yanukovych’s government. Mr. Manafort and Mr. Kilimnik discussed plans to conduct polling in connection with a possible bid by Mr. Klymenko for president of Ukraine in 2019, even though he was living in exile in Moscow and under European Union sanctions, according to three people briefed on their activities.

Another client was a party backed by Serhiy Lyovochkin, a television company owner who served as Mr. Yanukovych’s chief of staff. The party, known as Opposition Bloc, had fallen behind in paying Mr. Manafort’s invoices, according to Mr. Kilimnik and other people familiar with the situation, and the pair hoped to persuade Mr. Lyovochkin to turn the spigot back on.

.. The situation was on Mr. Kilimnik’s agenda when he headed to the United States within a month of Mr. Manafort’s joining the Trump campaign, telling associates in Kiev that he also hoped to meet with Mr. Trump and to potentially work with his campaign in some capacity.

A White House spokesman said Mr. Trump was introduced to thousands of people during the 2016 campaign. While it is possible he met with Mr. Kilimnik, “the president has no recollection of it,” the spokesman said.

Around the time of Mr. Kilimnik’s trip to the United States in spring 2016, Mr. Manafort directed Mr. Gates to transfer some polling data to Mr. Kilimnik, including public polling and some developed by a private polling company working for the campaign, according to a person with knowledge of the arrangement.

Mr. Manafort asked Mr. Gates to tell Mr. Kilimnik to pass the data to Mr. Lyovochkin and Mr. Akhmetov, the person said. Representatives for both Mr. Lyovochkin and Mr. Akhmetov said they neither requested nor received the data, and would have had no use for it.

Mr. Mueller’s team has focused on what appears to have been another discussion about polling data in New York on Aug. 2, 2016. A partly redacted court transcript suggests that Mr. Gates, who entered a plea agreement with the special counsel that requires his cooperation, may have told prosecutors that Mr. Manafort had walked Mr. Kilimnik through detailed polling data at a meeting that day in the cigar lounge of the Grand Havana Room in Manhattan.

.. The meeting also included a conversation about one Ukrainian “peace plan,” according to court filings.

Prosecutors contend that Mr. Manafort lied to them about the meeting and other interactions with Mr. Kilimnik. Those lies, a federal judge ruled, violated Mr. Manafort’s agreement to cooperate with prosecutors in exchange for consideration of a reduced sentence related to his work in Ukraine.

Mr. Manafort’s lies about his interactions with Mr. Kilimnik gave “rise to legitimate questions about where his loyalties lie,” Judge Amy Berman Jackson said.

It led to a guilty plea by Mr. Trump’s first national security adviser, Michael T. Flynn, for lying to the F.B.I. about conversations with the Russian ambassador related to the sanctions.

By the inauguration, investigators were focusing on Mr. Kilimnik and his connections to Mr. Manafort.

Yet Mr. Kilimnik again traveled to the United States for Mr. Trump’s inauguration, meeting in the Washington area with Mr. Manafort, according to Mr. Mueller’s team.

.. While the trip was not publicly revealed until nearly two years later, it was on the F.B.I.’s radar. Peter Strzok, the agent who oversaw the bureau’s investigation of Russian interference in the election until he was reassigned and later fired for sending text messages critical of Mr. Trump, expressed frustration when he learned that Mr. Kilimnik had come and gone without being interviewed by agents.

Yet Mr. Kilimnik again traveled to the United States for Mr. Trump’s inauguration, meeting in the Washington area with Mr. Manafort, according to Mr. Mueller’s team.

While the trip was not publicly revealed until nearly two years later, it was on the F.B.I.’s radar. Peter Strzok, the agent who oversaw the bureau’s investigation of Russian interference in the election until he was reassigned and later fired for sending text messages critical of Mr. Trump, expressed frustration when he learned that Mr. Kilimnik had come and gone without being interviewed by agents.

“Everything is completely falling off the rails,” Mr. Strzok wrote in a text to a colleague on Jan. 23, 2017 — three days after the inauguration — referring to the missed opportunity.

The month after the inauguration, Mr. Kilimnik and Mr. Manafort met again and discussed a poll being planned for Mr. Klymenko’s prospective presidential campaign, according to court filings and interviews.

The poll was not conducted, according to people familiar with the arrangement, and Mr. Klymenko ultimately did not run.

In previously unpublished messages to The Times around the time of that meeting, Mr. Kilimnik suggested that the escalating news coverage was making him a target in both Ukraine and Russia.

“After another surge of attention, I am sure I will finally get on the radar screen of the Russian authorities,” he wrote.

Mr. Manafort’s allies point out that Mr. Mueller’s team has not publicly presented any evidence that Mr. Kilimnik is a Russian agent. They argue that it is unlikely that he is an agent because he was able to travel freely to the United States and deal regularly with its officials. To buttress this case, Mr. Manafort’s lawyers requested and received records from the government showing that Mr. Kilimnik communicated with officials at the American Embassy in Kiev.

“If he was a Russian intelligence asset, then the State Department officials who met with him over the years should be under investigation,” Mr. Caputo said.